The present invention relates to encapsulation of microelectronic assemblies.
As described, for example, in U.S. Pat. No. 5,659,652; in U.S. patent application Ser. No. 08/532,235 filed Sep. 22, 1995; and in U.S. patent application Ser. No. 08/726,697, the disclosures of which are hereby incorporated by reference herein, microelectronic assemblies can be made with microelectronic elements such as a semiconductor chip having contact thereon and a flexible, sheet-like dielectric element having terminals thereon electrically connected to the contacts of the chip. The flexible dielectric element forms a top surface of the assembly, whereas the surface of the chip facing away from the flexible dielectric element forms the bottom surface of the assembly. For example, where the dielectric sheet overlies the front or contact-bearing surface of the chip, the back surface of the chip forms the bottom surface of the assembly. The assembly may also include a compliant layer such as an elastomer or gel disposed between the dielectric sheet and the chip to provide mechanical decoupling of the sheet and terminals from the chip.
As disclosed in the aforementioned patents and patent applications, such compliant layers can be made by providing a porous layer such as plurality compliant pads between the chip and the sheet, electrically connecting the terminals to the contacts of the chip and then encapsulating the resulting assembly with a curable liquid encapsulant so that the encapsulant penetrates into the porous layer and also covers the connections at the contacts on the chip. Upon curing of the encapsulant to form a compliant material, the encapsulant in the porous layer forms part of the compliant layer. In making assemblies of this nature, it is desirable to avoid applying encapsulant onto the top and bottom surfaces of the assembly. That is, it is desirable to avoid applying an encapsulant on the top surface of the dielectric sheet and on the terminals or on the surface of the chip defining the back surface of the assembly. It is also desirable to ensure that the encapsulant completely fills the porous layer, to provide a substantially void-free compliant layer in the final assembly.
Several useful methods of applying and curing the encapsulant in microelectronic assemblies have been proposed in commonly assigned, copending United States patents applications and patents. For example, as set forth in the aforementioned Ser. No. 08/532,235 application, the dielectric sheets incorporated in the assemblies typically include apertures or xe2x80x9cbond windowsxe2x80x9d which provide access to the contacts of the chip during the step of electrically connecting the terminals to the chip. After these connections have been made, a top cover layer may be applied over the top surface of the dielectric sheet. Typically, several assemblies are provided in a side by side arrangement so the same top cover layer lies on the top surfaces of several assemblies. The top cover layer seals the bond windows and also seals spaces between the dielectric sheets of adjacent assemblies. A bottom cover layer is provided over the bottom surfaces of the assemblies. The cover layers and assemblies are held in a fixture. A pocket in the fixture holds a curable liquid encapsulant. After the space between the cover layers is brought to subatmospheric pressure by connecting the fixture to a vacuum source, the fixture is tilted so as to pour the liquid encapsulant into the space between the cover layers whereupon the encapsulant is cured while the components remain in place in the fixture. The top and bottom cover layers retain the encapsulant and prevent it from contaminating the bottom surfaces of the chips or the top surfaces of the dielectric layers.
As described in the aforementioned U.S. Pat. No. 5,659,952 and U.S. patent application Ser. No 08/726,697 the encapsulant may be applied using a nozzle or a syringe around the periphery of each subassembly. For example, as shown in the ""697 application, a plurality of assemblies can be made using a single, unitary dielectric sheet element, commonly referred to as a xe2x80x9ctapexe2x80x9d which incorporates the dielectric sheets of several subassemblies. The chips may be attached to the tape and electrically connected to the terminals of the tape. At this point, a needle connected to an encapsulant dispenser is used to trace a pattern around the peripheries of the individual chips, so that the encapsulant flows into the space between each chip and the dielectric layer. A covering layer may be used to close the bond windows in the tape during this process. Also, during this process, the tape typically is held in a frame. In a further variant of this process, described in U.S. patent application Ser. No. 08/975,590, the disclosure of which is also incorporated by reference herein, the frame and tape are placed in a vacuum chamber, and the encapsulant dispensing operation is conducted inside the chamber, while the assembly is under vacuum. When the vacuum is released, and the chamber is brought to atmospheric or superatmospheric pressure, the pressure forces the encapsulant into the porous layer between the chip and the tape. Further, as described in U.S. patent application Ser. No. 09/012,590, filed Jan. 23, 1998, the disclosure of which is also incorporated by reference herein, it is convenient to use a frame to hold the tape during the assembly procedures. For example, the compliant pads used to form the porous layer may be applied to the tape while the tape is mounted on the frame and the chips may be mechanically attached to the pads and electrically connected to the terminals also while the tape is mounted on the frame.
Despite all of these advances in manufacture of microelectronic assemblies, still further improvement would be desirable.
One aspect of the invention provides methods of encapsulating micro-electronic assemblies. A method according to this aspect of the invention desirably includes the steps of providing the assemblies within an opening in a frame having top and bottom surfaces and providing top and bottom sealing layers sealingly connected to the frame extending across the opening of the frame. The top and bottom sealing layers together with the frame define an enclosed space encompassing the assemblies. Thus, the top and bottom sealing layers may extend over the surfaces of the assemblies, or one or both of the sealing layers themselves may define the surfaces of the assemblies. For example, where the assemblies are formed from semiconductor chips mounted to a flexible tape, one of the sealing layers may consist of the tape or may include the tape together with a separate cover layer overlying the tape. The frame, sealing layers and assemblies are engaged in a fixture as, for example, by clamping the frame and sealing layers between top and bottom elements of the fixture. While the frame is engaged in the fixture a liquid encapsulant is injected into the enclosed space, between the sealing layers and around the assemblies. The encapsulant is then cured. After curing, the assemblies are removed from the frame and severed from one another. Typically, the frame is a disposable unit, formed from a polymeric material, a fibrous material or combination of such materials and the frame is destroyed during the step of removing the assemblies from the frame. For example, the frame may be cut apart in the same operation used to sever the various assemblies from one another.
Because the frame and the sealing layers cooperatively confine the liquid encapsulant, the frame and sealing layers can be removed from the fixture after the encapsulant is injected. Thus, some frames which have already been injected with encapsulant may be cured while further frames are being filled with encapsulant. This avoids the need to occupy the encapsulation fixture for the extended period of time required to cure the encapsulant. Typically, this enclosed space is evacuated before the encapsulant is introduced. This allows filling of the encapsulant without formation of voids. For example, one of the sealing layers may incorporate a hole and a vacuum may be applied through such hole while the frame is in the encapsulation fixture. After the vacuum has been applied, a needle may be advanced into the hole and encapsulant may be injected through the needle. The needle is then withdrawn and the frame is removed from the encapsulation fixture.
As further discussed below, where one of the sealing layers incorporates a tape having electrically conductive components thereon, the step of providing the microelectronic assemblies frames and sealing layers may include the step of attaching several semiconductor chips to the tape while the tape is held in the frame. That is, the same frame used in the assembly procedure is also used in the encapsulation procedure, thus further simplifying handling. The sealing layers may be wholly or partially removable or else may include features which are intended to form part of the finished microelectronic assemblies. For example, the sealing layers may include thermally conductive structures which form heat spreaders in the finished micro-electronic assemblies; packaging elements which form protective structures in the finished assemblies; or both. Also, where the microelectronic assemblies incorporate optically active elements such as a photoemitter or photoreceiver, the sealing layers may incorporate light transmitting elements which act as a light window in the finished assembly. Such transparent elements may be refractive elements such as lenses.
An encapsulation fixture according to a further aspect of the invention may include a top fixture element defining a top element face and a passage open to the top element face, with a vacuum connection communicating with the passage. The fixture may also include a hollow needle having a distal tip slidably mounted in the passage for movement between a retracted position in which the distal tip is within the passage and an advanced position wherein the distal tip protrudes beyond the face. Further, the fixture may include an encapsulant dispenser connected to the hollow needle, a bottom fixture element defining a bottom face facing toward the top element face and a clamp engagable with the fixture elements for urging the fixture elements together. Such a fixture may be used in the aforementioned methods. The encapsulation fixture may include a bottom fixture having a central region and a peripheral region surrounding the central region. The peripheral region defines the bottom element face whereas a recess is provided in the central region. A floating plate is mounted in the recess of the bottom fixture. A clamp is arranged to urge the fixture elements towards one another so as to clamp a frame and sealing layers therebetween. The fixture is also provided with means for biasing the floating plate of the bottom fixture element upwardly, toward the top fixture element. As described further below, the floating plate maintains pressure on the microelectronic assemblies and helps to maintain the desired planar configuration of the sealing layers during encapsulant injection.
Yet a further aspect of the invention provides an encapsulation fixture including a plurality of frames as discussed below and a fixture, such as those discussed above, arranged to engage the frames in series, one after the other. The fixture has an encapsulant dispenser arranged to inject an encapsulant into the aperture of each frame while the frame is engaged between the elements of the fixture. The encapsulation system desirably includes a curing station such as a curing oven for holding frames after injection of the encapsulant. Yet a further aspect of the invention provides a semi-finished article including a frame as aforesaid having top and bottom surfaces and an aperture extending therethrough, top and bottom sealing layers sealingly connected to the frame and defining an enclosed space together with a plurality of microelectronic assemblies disposed in the enclosed space. The article may further include a cured or uncured encapsulant. Such a semi-finished article may be used in the processes as discussed above.